1. Field of the Invention
The present invention relates to a phase difference plate and a liquid crystal display device, and more particularly to a liquid crystal display device for improving the viewing angle characteristics of the display screen.
2. Description of the Related Art
The liquid crystal display device using nematic liquid crystal display cell has been hitherto applied widely in numerical value segment type display device such as clock and desktop calculator, and active elements such as thin film transistors are formed on the light transmittable substrate of liquid crystal display cell as the switching means for selectively driving the pixel electrode, and color filter layers of red, green and blue are provided as color display means, and depending on the twist angle of liquid crystal, (a) the active drive type twisted nematic (TN) liquid crystal display system disposing the nematic liquid crystal molecules by twisting 90 degrees, and (b) the multiplex drive type super twisted nematic (STN) liquid crystal display system making use of sharp steepness of transmissivity-liquid crystal applied voltage characteristic by defining the twist angle of nematic liquid crystal cell at 90 degrees or more are known.
In the latter (b) multiplex drive type STN liquid crystal display system, since peculiar coloring is present, the system of disposing optical compensation plates is considered useful for monochromatic display, and depending on the optical compensation plates it is classified into (b-1) two-layer type double super twisted nematic liquid crystal display system using liquid crystal cells twisted and disposed at a twist angle in reverse direction to the liquid crystal cell for display, and (b-2) the film added type liquid crystal display system disposing a film possessing optical anisotropy, and from the viewpoint of lightness of weight and cost, the latter (b-2) film added type liquid crystal display system is considered advantageous.
On the other hand, the former (a) active drive type TN liquid crystal display system is roughly classified into (a-1) normally black system for displaying black color in a state without voltage application to the liquid crystal layer (OFF state) by disposing the polarization directions of a pair of polarizers parallel to each other, and (a-2) normally white system for displaying white color in OFF state by mutually crossing the polarization directions orthogonally, and the normally white system is considered more useful from the viewpoints of display contrast, color reproduction and dependence of display on viewing angle.
In the conventional TN liquid crystal display device, however, since the refractive anisotropy is present in the liquid crystal molecules, and the liquid crystal molecules are disposed at inclination to the upper and lower electrode substrates, and the contrast of the display image varies depending on the viewing angle to observe, and the dependence on the viewing angle increases. In particular, as shown in the plan of the liquid crystal display cell in FIG. 3 or FIG. 19, when the viewing angle is inclined from the screen normal direction to the normal viewing angle direction 38, the display image is colored above a certain angle (this is called coloring phenomenon), or the black and white are inverted (reversal phenomenon). Or as the viewing angle is inclined in the anti-viewing angle direction 39, the contrast drops abruptly.
To improve such dependence on the viewing angle, it may be considered to compensate for the phase of light by placing a phase difference plate having the direction of one principal refractive index of index ellipsoid parallel to the normal direction of the surface, as shown in perspective view in FIG. 7, between the liquid crystal layer and polarizer, but even by using such phase difference plate, there is a limit for improving the reversal phenomenon in the normal viewing angle direction.